The rapid increase in nanotechnology has led to growing concerns on environmental effects and health risks of nanoparticles (NPs). Many studies investigated the adsorption of toxic pollutants on NPs; however, the interaction between heavy metals and natural organic matter (NOM) coated metal oxide NPs was scarcely studied. In this study, using humic acid (HA) as model NOM, the adsorption of Cd(II) on humic acid coated titanium dioxide (HA-TiO2) NPs was investigated. Solution parameters such as pH and salinity were investigated to exploit the mechanisms. Our results demonstrated that the adsorption isotherms of Cd(II) to both TiO2 and HA-TiO2 complied well with Freundlich model. q(e) values increased with pH increase, mainly due to electrostatic attraction, whereas q(e) values increased initially and then decreased at 100 mmol L-1 with salinity increase, mainly due to complexation and electrostatic effects. It is noteworthy that an overall trend of higher Cd(II) adsorption was observed on HA-TiO2 compared to that on TiO2, implying that HA coating might modify bioavailability of heavy metals in aquatic environment. The possible adsorption mechanisms in views of electrostatic interactions and covalent effects were interpreted, and the X-ray photoelectron spectroscopy (XPS) results also verified the possible mechanisms. (C) 2011 Elsevier Inc. All rights reserved.